Influence of the built environment on human mortality during extreme heat events

Background: Extreme heat events are the leading cause of weather-related death in the United States. Built environment features, such as the extent of tree shade cover, can modify temperatures within metropolitan areas exposing certain residents to more thermally stressful conditions. However, current heat warning systems only operate at the regional scale. This analysis investigates intra-city variability in heat-related mortality and the influence of built environment features. Method: Mortality records for 25 years were obtained for seven major U.S. cities. Daily mortality counts were aggregated at the zip-code scale, and a temperature–mortality response curve was calculated for each zip code with a generalized additive model approach tailored for small samples. Independent variables included measurements of demographics, building density, land cover, and satellite-derived surface temperatures. Principal components regression models were built to identify variables associated with high mortality zip codes. Results: Significant spatial variability in heat-related mortality was present within each study city. In Philadelphia, for example, although citywide mortality increased by 9.3% following days with extreme heat, high mortality was only evident in 10 of 48 zip codes. Across all cities, zip codes with higher mortality were associated with higher building densities, lower income and educational attainment, and higher surface temperatures. On average the models explained 35% of the spatial variability in mortality. Conclusions: Heat-related mortality varies significantly at smaller spatial scales than currently addressed by warning systems and high-mortality zones are co-located with built environment features known to contribute to heat island effects. Measures aimed at reducing the health burden from extreme heat could become more geographically focused on areas within cities that show the strongest response. This research can improve the efficiency of resource allocation with short-term intervention measures including heat warning systems and provides further justification for design improvements such as adding green space in urban areas.